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Clarifier

Environmental Engineering · FE Reference Handbook section

Environmental Engineering
9 formulas
10 exam-style examples
~60 min
All Environmental Engineering lectures

Handbook notes for this section

Definitions and conditions exactly as the handbook states them.

  • Typical Primary Clarifier Efficiency Percent Removal
  • Design Criteria for Sedimentation Basins
  • Overflow Rate Solids Loading Rate
  • Type of Basin Average Peak Average Peak Time (ft)

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1
Tank Volume — solve for tank volume — Clarifier

tank volume required for a target detention time in a storage tank Given flow rate (Q) = 16,350 m^3/day; hydraulic detention time (theta) = 7.3000 day, determine the tank volume (V) in m^3.

Given

  • flowrate(Q)=16,350m3/dayflow rate (Q) = 16,350 m^3/day
  • hydraulicdetentiontime(theta)=7.3000dayhydraulic detention time (theta) = 7.3000 day

Find

tank volume (V), in m^3

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except V is given, so isolate V symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 1 — schematic for Tank Volume — solve for tank volume — Clarifier

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for V:

    V=Q θV = Q\,\theta
  3. Step 3 — List the givens: flow rate (Q) = 16,350 m^3/day, hydraulic detention time (theta) = 7.3000 day.

  4. Step 4 — Substitute the given values:

    V=16350 7.3000V = 16350\,7.3000
  5. Step 5 — Evaluate:

    V = 119355\ \text{m^3}
  6. Step 6 — Check: returning V = 119,355 m^3 to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
V = 119355\ \text{m^3}

Why the other options are there

  • 238,710 — kept a factor of two that cancels in the correct rearrangement.
  • 59,678 — dropped that same factor in the other direction.
  • 131,291 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 2
Tank Volume — solve for flow rate — Clarifier (2)

tank volume sizing for an equalization basin Given hydraulic detention time (theta) = 5.1000 day; tank volume (V) = 149,316 m^3, determine the flow rate (Q) in m^3/day.

Given

  • hydraulicdetentiontime(theta)=5.1000dayhydraulic detention time (theta) = 5.1000 day
  • tankvolume(V)=149,316m3tank volume (V) = 149,316 m^3

Find

flow rate (Q), in m^3/day

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except Q is given, so isolate Q symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 2 — schematic for Tank Volume — solve for flow rate — Clarifier (2)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for Q:

    Q=VθQ = \dfrac{V}{\theta}
  3. Step 3 — List the givens: hydraulic detention time (theta) = 5.1000 day, tank volume (V) = 149,316 m^3.

  4. Step 4 — Substitute the given values:

    Q=1493165.1000Q = \dfrac{149316}{5.1000}
  5. Step 5 — Evaluate:

    Q = 29278\ \text{m^3/day}
  6. Step 6 — Check: returning Q = 29,278 m^3/day to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
Q = 29278\ \text{m^3/day}

Why the other options are there

  • 58,555 — kept a factor of two that cancels in the correct rearrangement.
  • 14,639 — dropped that same factor in the other direction.
  • 32,205 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 3
Tank Volume — solve for hydraulic detention time — Clarifier (3)

detention tank volume calculated from plant flow rate Given flow rate (Q) = 2,220 m^3/day; tank volume (V) = 218,303 m^3, determine the hydraulic detention time (theta) in day.

Given

  • flowrate(Q)=2,220m3/dayflow rate (Q) = 2,220 m^3/day
  • tankvolume(V)=218,303m3tank volume (V) = 218,303 m^3

Find

hydraulic detention time (theta), in day

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except theta is given, so isolate theta symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 3 — schematic for Tank Volume — solve for hydraulic detention time — Clarifier (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for theta:

    θ=VQ\theta = \dfrac{V}{Q}
  3. Step 3

    Listthegivens:flowrate(Q)=2,220m3/day,tankvolume(V)=218,303m3List the givens: flow rate (Q) = 2,220 m^3/day, tank volume (V) = 218,303 m^3
  4. Step 4 — Substitute the given values:

    θ=2183032220\theta = \dfrac{218303}{2220}
  5. Step 5 — Evaluate:

    θ=98.3347 day\theta = 98.3347\ \text{day}
  6. Step 6 — Check: returning theta = 98.3347 day to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
θ=98.3347 day\theta = 98.3347\ \text{day}

Why the other options are there

  • 196.7 — kept a factor of two that cancels in the correct rearrangement.
  • 49.1673 — dropped that same factor in the other direction.
  • 108.2 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 4
Tank Volume — solve for tank volume (case 2) — Clarifier (4)

tank volume required for a target detention time in a storage tank Given flow rate (Q) = 5,000 m^3/day; hydraulic detention time (theta) = 19.0000 day, determine the tank volume (V) in m^3.

Given

  • flowrate(Q)=5,000m3/dayflow rate (Q) = 5,000 m^3/day
  • hydraulicdetentiontime(theta)=19.0000dayhydraulic detention time (theta) = 19.0000 day

Find

tank volume (V), in m^3

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except V is given, so isolate V symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 4 — schematic for Tank Volume — solve for tank volume (case 2) — Clarifier (4)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for V:

    V=Q θV = Q\,\theta
  3. Step 3 — List the givens: flow rate (Q) = 5,000 m^3/day, hydraulic detention time (theta) = 19.0000 day.

  4. Step 4 — Substitute the given values:

    V=5000 19.0000V = 5000\,19.0000
  5. Step 5 — Evaluate:

    V = 95000\ \text{m^3}
  6. Step 6 — Check: returning V = 95,000 m^3 to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
V = 95000\ \text{m^3}

Why the other options are there

  • 190,000 — kept a factor of two that cancels in the correct rearrangement.
  • 47,500 — dropped that same factor in the other direction.
  • 104,500 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 5
Tank Volume — solve for flow rate (case 2) — Clarifier (5)

tank volume sizing for an equalization basin Given hydraulic detention time (theta) = 18.0000 day; tank volume (V) = 199,020 m^3, determine the flow rate (Q) in m^3/day.

Given

  • hydraulicdetentiontime(theta)=18.0000dayhydraulic detention time (theta) = 18.0000 day
  • tankvolume(V)=199,020m3tank volume (V) = 199,020 m^3

Find

flow rate (Q), in m^3/day

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except Q is given, so isolate Q symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 5 — schematic for Tank Volume — solve for flow rate (case 2) — Clarifier (5)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for Q:

    Q=VθQ = \dfrac{V}{\theta}
  3. Step 3 — List the givens: hydraulic detention time (theta) = 18.0000 day, tank volume (V) = 199,020 m^3.

  4. Step 4 — Substitute the given values:

    Q=19902018.0000Q = \dfrac{199020}{18.0000}
  5. Step 5 — Evaluate:

    Q = 11057\ \text{m^3/day}
  6. Step 6 — Check: returning Q = 11,057 m^3/day to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
Q = 11057\ \text{m^3/day}

Why the other options are there

  • 22,113 — kept a factor of two that cancels in the correct rearrangement.
  • 5,528 — dropped that same factor in the other direction.
  • 12,162 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 6
Tank Volume — solve for hydraulic detention time (case 2) — Clarifier (6)

detention tank volume calculated from plant flow rate Given flow rate (Q) = 11,260 m^3/day; tank volume (V) = 21,977 m^3, determine the hydraulic detention time (theta) in day.

Given

  • flowrate(Q)=11,260m3/dayflow rate (Q) = 11,260 m^3/day
  • tankvolume(V)=21,977m3tank volume (V) = 21,977 m^3

Find

hydraulic detention time (theta), in day

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except theta is given, so isolate theta symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 6 — schematic for Tank Volume — solve for hydraulic detention time (case 2) — Clarifier (6)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for theta:

    θ=VQ\theta = \dfrac{V}{Q}
  3. Step 3

    Listthegivens:flowrate(Q)=11,260m3/day,tankvolume(V)=21,977m3List the givens: flow rate (Q) = 11,260 m^3/day, tank volume (V) = 21,977 m^3
  4. Step 4 — Substitute the given values:

    θ=2197711260\theta = \dfrac{21977}{11260}
  5. Step 5 — Evaluate:

    θ=1.9518 day\theta = 1.9518\ \text{day}
  6. Step 6 — Check: returning theta = 1.9518 day to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
θ=1.9518 day\theta = 1.9518\ \text{day}

Why the other options are there

  • 3.9036 — kept a factor of two that cancels in the correct rearrangement.
  • 0.9759 — dropped that same factor in the other direction.
  • 2.1470 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 7
Tank Volume — solve for tank volume (case 3) — Clarifier (7)

tank volume required for a target detention time in a storage tank Given flow rate (Q) = 15,100 m^3/day; hydraulic detention time (theta) = 9.0000 day, determine the tank volume (V) in m^3.

Given

  • flowrate(Q)=15,100m3/dayflow rate (Q) = 15,100 m^3/day
  • hydraulicdetentiontime(theta)=9.0000dayhydraulic detention time (theta) = 9.0000 day

Find

tank volume (V), in m^3

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except V is given, so isolate V symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 7 — schematic for Tank Volume — solve for tank volume (case 3) — Clarifier (7)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for V:

    V=Q θV = Q\,\theta
  3. Step 3 — List the givens: flow rate (Q) = 15,100 m^3/day, hydraulic detention time (theta) = 9.0000 day.

  4. Step 4 — Substitute the given values:

    V=15100 9.0000V = 15100\,9.0000
  5. Step 5 — Evaluate:

    V = 135900\ \text{m^3}
  6. Step 6 — Check: returning V = 135,900 m^3 to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
V = 135900\ \text{m^3}

Why the other options are there

  • 271,800 — kept a factor of two that cancels in the correct rearrangement.
  • 67,950 — dropped that same factor in the other direction.
  • 149,490 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 8
Tank Volume — solve for flow rate (case 3) — Clarifier (8)

tank volume sizing for an equalization basin Given hydraulic detention time (theta) = 0.9000 day; tank volume (V) = 79,559 m^3, determine the flow rate (Q) in m^3/day.

Given

  • hydraulicdetentiontime(theta)=0.9000dayhydraulic detention time (theta) = 0.9000 day
  • tankvolume(V)=79,559m3tank volume (V) = 79,559 m^3

Find

flow rate (Q), in m^3/day

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except Q is given, so isolate Q symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 8 — schematic for Tank Volume — solve for flow rate (case 3) — Clarifier (8)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for Q:

    Q=VθQ = \dfrac{V}{\theta}
  3. Step 3 — List the givens: hydraulic detention time (theta) = 0.9000 day, tank volume (V) = 79,559 m^3.

  4. Step 4 — Substitute the given values:

    Q=795590.9000Q = \dfrac{79559}{0.9000}
  5. Step 5 — Evaluate:

    Q = 88399\ \text{m^3/day}
  6. Step 6 — Check: returning Q = 88,399 m^3/day to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
Q = 88399\ \text{m^3/day}

Why the other options are there

  • 176,798 — kept a factor of two that cancels in the correct rearrangement.
  • 44,199 — dropped that same factor in the other direction.
  • 97,239 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 9
Tank Volume — solve for hydraulic detention time (case 3) — Clarifier (9)

detention tank volume calculated from plant flow rate Given flow rate (Q) = 3,320 m^3/day; tank volume (V) = 377,106 m^3, determine the hydraulic detention time (theta) in day.

Given

  • flowrate(Q)=3,320m3/dayflow rate (Q) = 3,320 m^3/day
  • tankvolume(V)=377,106m3tank volume (V) = 377,106 m^3

Find

hydraulic detention time (theta), in day

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except theta is given, so isolate theta symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 9 — schematic for Tank Volume — solve for hydraulic detention time (case 3) — Clarifier (9)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for theta:

    θ=VQ\theta = \dfrac{V}{Q}
  3. Step 3

    Listthegivens:flowrate(Q)=3,320m3/day,tankvolume(V)=377,106m3List the givens: flow rate (Q) = 3,320 m^3/day, tank volume (V) = 377,106 m^3
  4. Step 4 — Substitute the given values:

    θ=3771063320\theta = \dfrac{377106}{3320}
  5. Step 5 — Evaluate:

    θ=113.6 day\theta = 113.6\ \text{day}
  6. Step 6 — Check: returning theta = 113.6 day to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
θ=113.6 day\theta = 113.6\ \text{day}

Why the other options are there

  • 227.2 — kept a factor of two that cancels in the correct rearrangement.
  • 56.7931 — dropped that same factor in the other direction.
  • 124.9 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

Example 10
Tank Volume — solve for tank volume (case 4) — Clarifier (10)

tank volume required for a target detention time in a storage tank Given flow rate (Q) = 6,100 m^3/day; hydraulic detention time (theta) = 7.1500 day, determine the tank volume (V) in m^3.

Given

  • flowrate(Q)=6,100m3/dayflow rate (Q) = 6,100 m^3/day
  • hydraulicdetentiontime(theta)=7.1500dayhydraulic detention time (theta) = 7.1500 day

Find

tank volume (V), in m^3

Start with the thinking

  • The governing relation printed in this handbook section is Tank Volume.
  • Everything except V is given, so isolate V symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • Tank volume for a treatment basin is the product of flow rate and required hydraulic detention time.
V = tank volume

Figure 10 — schematic for Tank Volume — solve for tank volume (case 4) — Clarifier (10)

Step-by-step solution

  1. Step 1 — State the governing relation:

    V=Q×θV = Q \times \theta
  2. Step 2 — Rearrange symbolically for V:

    V=Q θV = Q\,\theta
  3. Step 3 — List the givens: flow rate (Q) = 6,100 m^3/day, hydraulic detention time (theta) = 7.1500 day.

  4. Step 4 — Substitute the given values:

    V=6100 7.1500V = 6100\,7.1500
  5. Step 5 — Evaluate:

    V = 43615\ \text{m^3}
  6. Step 6 — Check: returning V = 43,615 m^3 to

    V=Q×θV = Q \times \theta

    reproduces the given quantities, and both sides carry the same units.

Answer:
V = 43615\ \text{m^3}

Why the other options are there

  • 87,230 — kept a factor of two that cancels in the correct rearrangement.
  • 21,808 — dropped that same factor in the other direction.
  • 47,977 — rounded an intermediate value before the final step.

Reference: FE Handbook — Tank Volume

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